The present invention relates to glasses within the system B.sub.2 O.sub.3 --SiO.sub.2 --La.sub.2 O.sub.3 --ZrO.sub.2 --Nb.sub.2 O.sub.5 and/or Ta.sub.2 O.sub.5, of refractive indices of 1.86 to 1.94 and Abbe numbers equal to or greater than 30.
Glasses which in the widest possible sense may potentially be composed similarly to the glasses according to this invention are described in Japanese OS No. 7 421 408 and in German OS No. 22 57 635. The glasses of these very broadly drawn applications differ from the present invention in that, within the claimed ranges, either the demanded high refractive indices above 1.86 are unattained (German OS No. 22 57 635) or that ZnO must be present in percentages of more than 5% by weight (JP-OS No. 7 421 408 and German OS No. 22 57 635). Because of the insufficient refractive index-improving effect of ZnO, the latter requirement leads to insufficiently high refractive indices or, necessarily, to a decrease in the vitrifier content of B.sub.2 O.sub.3 and SiO.sub.2 with the consequence of insufficient devitrification stability.
Glasses according to JP-PS No. 7 506 326 must necessarily contain Gd.sub.2 O.sub.3 in percentages from 2-50 wt. %. They are thus fundamentally distinguished from the glasses according to the present invention. Due to this Gd.sub.2 O.sub.3 percentage they are more expensive also.
German AS No. 10 70 794 describes glasses having refractive indices up to 1.87; higher refractive indices cannot be obtained according to this disclosure because the La.sub.2 O.sub.3 content is limited to 40 wt. %. German OS No. 20 20 421 likewise describes high-refraction glasses; however, it fails to disclose the extremely favorable effect of the presence of SiO.sub.2 and, secondly, these known glasses require high percentages of BaO designed to render them suitable for the desired application (core glass for fibers). There is no reference to Abbe-values which are of particular importance in optical glasses.